In order to measure, within a wind tunnel, various aerodynamic coefficients of spinning projectiles such as flechettes, instrumented spinning models have to be used in conjunction with a six degrees of freedom strain gauge type sting balance.
Unlike spin-stabilized projectiles where the spin motion is forced against the airflow by inertia forces, and where the spin rate is slowly damped by aerodynamic forces, flechette projectiles have their spin motion induced and stabilized by aerodynamic forces on their fins.
Consequently, when undertaking wind tunnel tests, the spin motion of a spin stabilized projectile (without fins) has to be transmitted by mechanical means to the model and the spin motion of a flechette projectile model must be left free so that spinning can occur at a natural spin rate in order to reproduce flow around the fins and obtain reliable data.
This condition does not cause any particular problem when a blow-down wind tunnel is used, because of the high level of dynamic pressures involved. At the beginning of a wind tunnel run, the model can freely accelerate to spin while leaving long enough time to perform measurements.
However when using an indraft wind tunnel, where the available dynamic pressure may be 30 times less than with a blow-down wind tunnel, the air flow build up time to accelerate the model up to spinning speed is insufficient.
In such a case, the model is accelerated to the correct spin rate level before a wind tunnel run, by a mechanical means which is disengaged just before the measurements are made. The delay between disengaging the mechanical means and taking the measurements is required to allow the spin rate to stabilize naturally.
This invention is related to an electrical spin rig which is particularly suitable for accelerating flechette models to the desired spin rate before a test run is made particularly but not exclusively in an indraft wind tunnel.
There is no way known in the prior art to test in an indraft wind tunnel, very small spinning models of flechette type projectiles which are for instance normally less than 0.75 inches in diameter.
In the case of much larger model sizes which are possibly more than 2 inches in diameter, an air turbine is used to spin the models. The present state of the art system consists of an air turbine and gear reduction situated within a conical part of the sting together with a mechanism to engage or disengage the air turbine at will so that the model can spin freely.
The only known facility of this type is at NASA, and it has a very large spin rig used for finned rocket models of about 10-in. in diameter and 9 feet in length. No publications on this item are known.
The only known disclosure in this field is found in the article "Spin Force Testing of High Finness Ration Configurations" by J. M. Cooksey, Manager, Wind Tunnel Laboratories, Oct. 2nd, 1986, LTV Aerospace and Defence Company, Vought Missiles and Advanced Programs Divisions, Dallas, Tex. This publication discloses a spin rig for smooth (no fins) models of high length to diameter ratios (about 30). The drive motor which is an air turbine would not be powerful enough to spin a model fitted with fins. The turbine is not contained inside the model but is integral with the sting at the model base thereof and can be disengaged at will. The model is about 2 inches in diameter and 5 feet long. It is not possible to utilize this spin rig with a model having a diameter less than this 2 inches especially when fitted with fins as it is not possible to obtain the power to be able to spin such a finned model.
The present state of the art using an air turbine to spin a wind tunnel flechette model is not easily applicable to model size of about 0.75 inch in diameter and the associated sting size.
Because of the intrinsic physics of air turbines, the necessary power (about 1100 watts) would impose very high revolutions to the turbine (more than 120,000 RPM) which leads to complicated technology like oil mist lubrication, overspeeding bearings and problems associated with compressed air between the gear teeth. The resulting mechanism would therefore be very fragile and prone to damage.
Turbine exhaust air ports would also have to be proportionally very large (2 in.sup.2.) and would lead to a weak sting support from a structural point of view. This is undesirable because of the high length/diameter ratio of a typical flechette model that requires a strong support sting which will not flex due to the large pitching moments encountered.
It is also difficult to incorporate an air turbine into a servo system, whereas a D.C. electric motor can be controlled so that the torque polarity can be momentarily reversed to slow down the spin rate during servo feedback control.